"Polar bears have indeed lost a lot of their past genetic diversity, and because of this, they are very likely more sensitive to climate-change threats today.”

Charlotte Lindqvist, Assistant Professor

Biological Sciences

An analysis of newly sequenced polar bear genomes is providing
important clues about the species’ evolution, suggesting that
climate change and genetic exchange with brown bears helped create
the polar bear as we know it today.

The international study, led by the Pennsylvania State
University and UB, found evidence that the size of the polar bear
population fluctuated with key climatic events over the past
million years, growing during periods of cooling and shrinking in
warmer times.

The research also suggests that while polar bears evolved into a
distinct species as many as 4-5 million years ago, the animals may
have interbred with brown bears until much more recently.

These intimate relations may be tied to changes in the
Earth’s climate, with the retreat of glaciers bringing the
two species into greater contact as their ranges overlapped, says
Charlotte Lindqvist, the study’s senior author and UB
assistant professor of biological sciences.

“Maybe we’re seeing a hint that in really warm
times, polar bears changed their lifestyle and came into contact,
and indeed interbred, with brown bears,” says Stephan
Schuster, co-lead author, a professor of biochemistry and molecular
biology at Penn State, and a research scientist at Nanyang
Technological University in Singapore.

The findings were published online Monday in the early edition
of the Proceedings of the National Academy of Sciences. The study
is the most extensive analysis to date of polar bear DNA,
scientists say.

The research team, representing 13 institutions in the U.S.,
Canada, Europe and Asia, as well as Mexico’s Laboratorio
Nacional de Genomica para la Biodiversidad (Langebio), sequenced
and analyzed the nuclear genomes of 28 different bears, with many
DNA samples provided by the U.S. Geological Survey and the
Norwegian Polar Institute.

“We generated a first-rate set of data, including deep
sequence coverage for the entire genomes of a polar bear, three
brown bears and a black bear, plus lower coverage of 23 additional
polar bears, including a 120,000-year-old individual; very few
vertebrate species have such comprehensive genomic resources
available,” Schuster notes.

Using this vast amount of data, the scientists discovered that
polar bears are actually an older species than previously
thought—indeed, far more ancient than suggested by a recent
study that placed the species’ age at 600,000 years old. That
analysis looked only at small segments of DNA.

“We showed, based on a consideration of the entire DNA
sequence, that earlier inferences were entirely misleading,”
said study co-lead author Webb Miller, a Penn State professor of
biology and computer science and engineering. “Rather than
polar bears splitting from brown bears a few hundred thousand years
ago, we estimate that the split occurred 4-5 million years
ago.”

“This means polar bears definitely persisted through
warming periods during Earth’s history,” Lindqvist
says. She cautions, however, that the species’ endurance over
several million years doesn’t guarantee its future
survival.

To model historical populations of the polar bear, the
scientists used computer simulations to analyze a deeply sequenced
polar bear genome.

“This is the first time we can see, from their genes, how
the population history of polar bears tracked Earth’s climate
history,” Lindqvist says. “We see an increase in polar
bears at the end of the Early Pleistocene as the Earth became much
colder, and a continuous decline in the size of the population
during warmer times.

“We also found, perhaps unsurprisingly, that polar bears
occur in much smaller numbers today than during prehistory,”
she continues “They have indeed lost a lot of their past
genetic diversity, and because of this, they are very likely more
sensitive to climate-change threats today.”

Discrepancies between the estimated age of polar bears in the
new study and past studies could be explained by interbreeding
between polar bears and brown bears since the species split from
each other.

The new analysis uncovered more genetic similarities than
previously known between polar bears and ABC brown bears, an
isolated group from southeastern Alaska—suggesting that these
animals have exchanged genes since becoming separate species.

“The ABC brown bears’ mitochondrial sequences are
much more like polar bears’ than like other brown
bears’,” Miller says. “This made us wonder what
other parts of their genomes are ‘polar-bear-like.’ We
mapped such regions, which constitute 5 to 10 percent of their
total DNA, onto the genomes of two ABC brown bears. As such,
brown/polar bear hybridization, which has been observed recently in
Arctic Canada, has undoubtedly contributed to shaping the modern
polar bear’s evolutionary story.”

This intermingling between species is just one interesting
finding emerging from the enormous trove of data that the PNAS
study produced. Another question that the research is beginning to
address: What makes a polar bear a polar bear?

Polar bears have genetic differences from brown bears that let
them survive in an Arctic climate with very different diets and the
new study identified genes that may be responsible for traits such
as polar bears’ pigmentation and the high fat content of
their milk.

This study received financial support from Penn State, the UB
College of Arts and Sciences, U.S. Geological Survey’s
Changing Arctic Ecosystem Initiative, U.S. Fish and Wildlife
Service, Ontario Ministry of Natural Resources in Canada, Alaska
Department of Fish and Game, the National Institutes of Health and
the National Science Foundation.